XXVI. ANIMALS OF THE PAST 



The signs that indicate the existence of prehistoric ani- 

 mals. — There are signs, or indications, scattered over the 

 earth that denote the past existence of a great host of ani- 

 mals different from those now Uving. This vast assemblage 

 of curious and interesting forms was born, lived, and died 

 before the present, species came to inhabit the land and seas. 

 The signs left by these prehistoric creatures consist of bones, 

 teeth, footprints, shells, and skeletons that are found in the 

 rocky structure of the earth, in quarries, railway cuts, beds 

 of streams, and in many other places. We call these signs 

 by one comprehensive term, fossils. It is by means of fos- 

 sils that we have been able to learn something of this mighty 

 and ancient host; and any discussion of the animal king- 

 dom would be incomplete without mention of these bygone 

 forms. 



Eras of the earth. — Briefly, geologists divide the past 

 history of the earth into four great chapters, or, as they are 

 called, eras. These are in their order, beginning with the 

 oldest, as follows: archean, paleozoic, mesozoic, and ceno- 

 zoic. From a zoological standpoint the archean era is 

 characterized by a noticeable lack of fossils or any absolute 

 indications of animal hfe. The paleozoic era is prominent 

 as an age when invertebrates existed, while the mesozoic is 

 characterized by its great number of reptiles. Finally, the 

 cenozoic, or last era, is known as the age of mammals. 



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